Conventionally, a technique for performing optical modulation using a material of which the refractive index is changed by an electric field, such as lithium niobate (LiNbO3) or the like. Japanese Patent Application Laid Open Gazette No. 2000-313141 (Document 1), for example, discloses an optical modulator in which a plurality of electrode elements are arrayed in one direction on one main surface of a thick plate-like electro-optic substrate. In the optical modulator, an electric field is produced inside the substrate by applying voltage between adjacent electrode elements, and this enables diffraction of light travelling inside the electro-optic substrate.
Actually, inside the electro-optic substrate, the range (in a thickness direction) in which the refractive index is changed depends on the voltage applied between the electrode elements. In the optical modulator of Document 1, the electrode elements are elongated in a traveling direction of light and light which enters the electro-optic substrate from one end surface thereof and travels thereinside is caused to enter the electro-optic substrate at a small angle (at a large incident angle) with respect to a main surface on which the electrode elements are formed and to be totally reflected on the main surface, to thereby produce a phase difference required to diffract the light. At that time, a relatively large voltage of 60V to 100V needs to be applied between the electrode elements.
“Nanosecond Response of Bragg Deflectors in Periodically Poled LiNbO3” by Harald Gnewuch, Christopher N. Pannell, Graeme W. Ross, Peter G. R. Smith, and Harald Geiger, IEEE PHOTONICS TECHNOLOGY LETTERS, DECEMBER 1998, VOL. 10, NO. 12, pp. 1730-1732 (Document 2) discloses a technique in a deflector, in which one electrode is provided on each of the upper and lower surfaces of a substrate of lithium niobate having a periodically-poled structure and by changing voltage to be applied between the respective electrodes on the upper and lower surfaces, light entering the substrate is caused to exit as zeroth order beams or exit as first order diffracted beams due to Bragg diffraction.
In order to achieve high-speed modulation in an optical modulator, however, it is necessary to reduce the voltage applied between electrode elements. If such an optical modulator is used for an image recording apparatus for recording an image on a recording material by irradiating the recording material with light, it is necessary to shorten the distance between the electrode elements in order to respond to high definition of the image, and also in this case, it is necessary to reduce the voltage applied between the electrode elements. In Document 1, however, a relatively large voltage (e.g., 100V) needs to be applied between the electrode elements for the purpose of producing a phase difference required to diffract the incident light, and it is therefore impossible to reduce the voltage applied between the electrode elements. Further, for the purpose of preventing discharge (leakage) between the electrode elements, it is also impossible to shorten the distance between the electrode elements.